The development of small, efficient optical transmission lines such as optical fibers, has lead to widespread use of optical communication in many applications requiring, e.g., long distance, high data rate communication such as telecommunications. Optical fibers typically include a transparent core surrounded by a transparent cladding material having a refractive index lower than that of the core. Fiber optic transmission lines provide low cost, compact, low EMI (electromagnetic interference), and high-speed data transmission over significant distances.
In general, optical communications devices can be constructed using optical benches, or SiOB (silicon-optical bench) structures to couple optical fibers, for example, to otpoelectronic components (transmitter and/or receiver) and associated driver/control integrated circuit (IC) chips. For example, an optoelectronic transmitter package comprises a transmitter (optical emitter) interfaced with a connector to optical fibers. In general, a transmitter includes a semiconductor die having light sources that emit light signals in response to electrical signals, which light signals are then transmitted through optical fibers coupled thereto.
Optoelectronic transmitter devices may employ one of a variety of light emitting diodes (LEDs) and lasers as the light sources. For instance, a vertical-cavity surface emitting laser (VCSEL) is a specialized laser diode that has been developed to provide improved efficiency and increased data speed in fiber optic communication. VCSELs are good candidates for building parallel optics communication modules since their power dissipation is low and they can be manufactured in arrays or matrix form. A VCSEL emits light in the direction that is perpendicular to the surface of the wafer.
Furthermore, an optoelectronic receiver package generally comprises a receiver (optical detector) interfaced with a connector to optical fibers. A receiver comprises a semiconductor die with light detectors (e.g., photodiodes) that receive light signals from optical fibers and convert the light signals into electrical signals. In addition, optical benches can be used for constructing optoelectronic transceiver packages comprising a transmitter and receiver interfaced with optical fibers, for example.
When designing optical communications modules and systems using OE receiver and/or transmitter chips, it is generally desirable to position optical fibers and optoelectronic chips parallel to the surface of a PCB (printed circuit board) or an optical bench. In this manner, a plurality of PCBs (having optical fibers and devices mounted thereon) can be closely spaced in parallel to each other, for example.
However, when optical fibers are disposed parallel to the surface of optoelectronic chips that emit light or detect light perpendicular to the wafer surface (and, consequently, perpendicular to the core axis of the optical fiber), there are various coupling techniques that may be employed for coupling light between the optical fibers and the light sources or light detectors.
One coupling technique is to provide a 90 degree bend in the fiber so that the end of the fiber can be effectively butted to the light source or detector. This method requires a large spacing between PCBS, for example, because of the large minimum bending radius of the optical fibers, and results in increased light loss, which may not be acceptable for various applications.
Another method is to use flex connections. For instance, a VCSEL bar can be bonded to a flexible electrical connector (flex) that is bent by 90 degrees so that the light can be coupled to the fibers. A flex connector may be comprised of metal conductors embedded in a polyimide film. Due to the mechanical properties of the flex material, a bending radius of at least one centimeter is required to obtain a 90 degree bend, which makes the wires on the flex too long to accommodate high speed signals. Moreover, the coupling of the VCSEL to the fiber requires additional optic (such as lenses) since the fiber cannot be brought close enough to the VCSEL.
Other coupling techniques include “side-coupling” methods wherein an end portion of optical fiber is disposed adjacent to the light source/detector, and wherein light emitted from a light source perpendicular to the axis of the core is coupled into the optical fiber using a mirror structure disposed near the end of the fiber, or wherein light emitted from the fiber perpendicular to the light receiving surface of a detector is coupled to the detector using a mirror. In other embodiments, an angular facet can be formed on the end of the fiber, which acts as a reflective surface (either with a reflective material formed therein or by TIR (total internal reflection) to couple light between the angular fiber end OE device aligned thereto.
For example, FIGS. 1a and 1b illustrate a conventional side-coupling method for coupling light to and from an optical fiber from the side thereof by providing an acute angular cut along the end of the optical fiber. As shown in FIGS. 1a and 1b, an optical fiber (1), which comprises a fiber core (2) surrounded by a transparent cladding material (3), comprises a reflective acute angular facet (4) formed on an end thereof, which serves as a mirror for side-coupling light to/from an optoelectronic device (5) (e.g., a top or bottom surface emitter light source, detector). The optical fiber (1) is brought in parallel to the surface of an optoelectronic device (5) (or parallel to a module, chip, optical bench surface, etc., comprising the device (5)), the surface being substantially parallel to fiber axis (6), such that the optoelectronic device (5) is aligned adjacent the side of the optical fiber (1) opposite an inner facing surface of the reflective facet (4). A reflective material is deposited on an outer surface of the facet (4).
With the side-coupling method depicted in FIGS. 1a and 1b, the light emitted in a plane perpendicular to the fiber central axis (6) is preferably reflected into the optical fiber core (2) substantially parallel to the fiber central axis (6). Further, the light traveling within the fiber parallel to the fiber axis (6) toward the reflective angular cut (4) is reflected out of the fiber core (2) through the cladding layer (3) to a detector. As illustrated in FIG. 1a, the curved fiber optic cladding material (3), which is disposed between the optoelectronic device (5) and the inner surface of the cut end (4) of the fiber core (2), acts as a cylindrical lens to partially collimate the light from a light source into the fiber core (2) as well as reduce the divergence of the light propagating from the fiber toward the detector (5).
The use of optical mirrors on an optical bench or reflective facets formed on the fibers can add to the time, cost and complexity of manufacturing optoelectronic packages. Furthermore, the use of additional components such as mirrors, for example, can add more factors that decrease the accuracy of alignment of the OE device and fibers to provide sufficient coupling of light and increase optical cross-talk.
As the operating speed of optical communications systems increases, lower optical coupling losses are required. Thus, it is highly desirable to develop devices and methods for packaging optoelectronic devices and optical fibers, which provide efficient and accurate alignment for directly coupling OE devices and fibers, as well as compact designs for purposes of high-speed operation and space efficiency.